A porous honeycomb structure has been widely used as a filter for trapping and removing particulate matter included in dust containing fluid such as diesel engine exhaust gas, or a catalyst carrier for carrying a catalyst component purifying harmful substances in exhaust gas. As a constituent material of such a honeycomb structure, fire-resistant particles such as silicon carbide (SiC) particles have been used.
The present inventors proposed a porous honeycomb structure (Si-bonded SiC porous object) including fire-resistant particles (silicon carbide particles) as an aggregate and metal silicon as a binder bonding the fire-resistant particles to one another, and a method for producing the same (See Patent Document 1). The honeycomb structure disclosed in Patent Document 1 can be produced at a relatively low firing temperature with high yield, has a high thermal conductivity, a sufficient porosity, and a high-specific surface area, and can be appropriately used as a filter or a catalyst carrier.
In the silicon carbide porous object, there are problems to be solved according to a special environment or a process method.
For example, when metal silicon is heated in a low-oxygen atmosphere or a reduction atmosphere, Si is volatilized or SiO is volatilized as shown in the following formula (1). When Si or SiO in the gas state causes oxidation reaction, violent heat-generation is accompanied with the reaction.Si+1/2O2→SiO↑  (1)
The generated SiO gas is combined with oxygen in an atmosphere to generate Si2 fiber, which precipitates on the surface. For this reason, a color in the part where such the oxidation reaction has occurred changes to white by the generation of SiO2 fiber.
To solve the aforementioned problem, for example, there has been proposed a silicon carbide porous object in which a phase including oxygen is formed on the surface or in the vicinity of silicon carbide particles and/or metal silicon (see Patent Document 2). In the silicon carbide porous object described in Patent Document 2, a melting point of the phase including oxygen on the surface or in the vicinity of silicon carbide and/or metal silicon is at most 1700° C. When the other metal is mixed therein, the melting point decreases and oxidation resistance deteriorates.
To solve the aforementioned problems, it has been examined that silicon carbide and/or metal silicon, particularly, metal silicon is coated with a compound (e.g., corundum) including alumina having a melting point higher than that of the phase including oxygen on the surface or in the vicinity of vicinity of silicon carbide and/or metal silicon.
Patent Document 1: JP-A 2002-201082
Patent Document 2: JP-A 2002-154882